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The good news is that juvenile rats aren't the main consumer of Vertex Pharmaceuticals' (Nasdaq: VRTX) Kalydeco. The bad news is the cataracts they get when taking Kalydeco could be a bad sign.

The FDA issued an alert yesterday pointing out the cataracts that formed in young rats treated with the cystic fibrosis drug. The agency is adding the finding to Kalydeco's label, but it isn't clear whether it's relevant to humans because eye development of humans and rats is different.

So far Vertex hasn't gotten any reports of patients getting cataracts in the clinical trials that supported its approval or from patients taking Kalydeco since it hit the market earlier this year. The drug is approved for patients as young as 6 years old, but Vertex is hoping to expand the approval to children as young as 2.

The only real way to know if this is an issue is to look at its use in humans; the FDA is requiring Vertex to run a study examining the eyes of patients up to 11 taking Kalydeco. After 11, children's eye development is generally complete.

While that study is going on, I doubt the rat findings will have much of an effect on the use of Kalydeco. The drug is revolutionary in that it treats the underlying cause of cystic fibrosis rather than just treating the symptoms like Novartis' (NYSE: NVS) Tobi or Gilead Sciences' (Nasdaq: GILD) Cayston. The FDA suggests that patients talk to their doctor, but the known efficacy would seem to trump the unknown potential side effect for most patients.

My guess is that this is much ado about nothing, but we won't know for a while. Unfortunately having a safety issue hanging over the biotech could negatively affect the shares until the issue is worked out. The finding also might hold up Vertex's plans to expand into younger patients, which would slow the sales launch.

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